1. Field of the Invention
The present invention relates to silicon capacitive micro-sensors. Such micro-sensors are, for example, pressure sensors, acceleration sensors, and so forth.
2. Discussion of the Related Art
Over the last years, new silicon micro-sensors have been developed, taking advantage of the silicon etching techniques developed for the manufacturing of semiconductor electronic components. Initially, such micro-sensors were formed by the combination of suitably etched silicon plates and thin glass plates serving as airtight chamber or as separating insulating plates inserted between the silicon plates, these glass plates bearing various metal electrode patterns.
Nowadays, the trend is to develop micro-sensors entirely made of silicon, without any glass plate having an active function. Schematic examples of such sensors are shown in FIGS. 1 and 2.
FIG. 1 shows an acceleration sensor; FIG. 2 shows a pressure sensor. Each of these sensors includes a central silicon plate 1 sandwiched between the external silicon plates 2 and 3. Insulation between the plates is ensured by a first insulating stripe, usually a silicon oxide layer, 5, between plates 1 and 2, and a second insulating stripe, 6, between plates 1 and 3. Such frame insulating stripes are disposed between adjacent plates, along the edges of the latter. Oxide layers are grown or deposited on one of the adjacent plates. Once the three plates are assembled, welding is carried out by annealing at a temperature within the range of 900.degree. to 1100.degree. C. To achieve this purpose, experiment shows that the silicon and silicon oxide surfaces facing each other must have a very slight roughness, for example lower than 0.5 nm. The external silicon plates 2 and 3 define between themselves and with the frame portion of the silicon plate 1 a region in which a controlled atmosphere is contained, such as an atmosphere below atmospheric pressure, referred to as a vacuum.
The above description commonly applies to the micro-sensors of FIGS. 1 and 2.
In the example of FIG. 1, which is a schematic cross-sectional view of an accelerometer, the central silicon plate 1 is etched before being assembled in order to include a frame and a central plate or inertia block 8 fixed to the frame by thin suspension rods 9. A single rod is shown in the schematic cross-sectional drawing of FIG. 1. Conventionally, two-rod or four-rod suspension systems are used. The external plates 1 and 3 delineate a vacuum cavity with the frame formed at the periphery of the central plate. The capacitance variations between the upper surface of the inertia block and the silicon plate 3 are detected and also, if required, between the lower surface of the inertia block and the lower plate 2. When the device is subject to acceleration, inertia block 8 moves with respect to the whole device, causing the above-mentioned capacities to vary. Additionally, an electrostatic control is generally provided in order to maintain the inertia block in place by applying a d.c. electric field; it is then the error signal which indicates the capacity variation.
FIG. 2 schematically shows a structure forming a pressure sensor. The lower plate 2 is etched in order to form a thin diaphragm 11. Plate 1 is etched to form a stud 12 contacting the diaphragm. A silicon strip 13 extends between stud 12 and the frame of plate 1. Again, the area delineated by the upper plate, the lower plate and the frame of the central plate defines a vacuum cavity. Variations of the external pressure deform diaphragm 11 and create stresses in strip 13. This stress variation causes variation of the resonance frequency of the capacitive resonator formed by strip 13 (vibrating beam) and the opposing surface of plate 3. Preferably, the internal side of plate 3 is etched in order to form a protruding stripe facing strip 13.
In the example of FIG. 1, electrodes 21, 22 and 23 must be coupled to plates 1, 2 and 3, respectively. In the example of FIG. 2, only electrodes 21 and 23 are required.
The drawback of these prior art structures, directly measuring either a capacity or a resonance frequency, is that measurements are substantially impaired by the presence of high stray capacities. Such stray capacities mainly correspond to the capacities between the frame formed in plate 1 and the opposed portions of the upper and/or lower plate, the dielectric of these capacities being formed by the silicon oxide insulating layers 5 and 6. The value of these stray capacities is not only high but also variable. This variation may more particularly result from deposition of polluting material or dampness on the lateral surfaces of the device at the sides of the insulating layers 5 and 6. Additionally, in practice, the manufacturing of a micro-sensor as the one shown in FIG. 1 or 2 results from a collective process, each plate being initially part of a silicon wafer, the separation into individual sensors being made after the end assembling operations and, if required, after the contact operations. Generally this separation is made by sawing off and, during the sawing step, silicon particles can be moved to the above-mentioned surface areas, which increases the value of the stray capacities and may even short two silicon plates.